Add basic support for Pandora

Add support for Pandora

Things working
- Start/stop Pandora
- Play, skip, vote

Things not working
- Sound control -- I'm not sure what to do with the CastPlatform. Am I support to just write something in a websocket? Or do I have to tell Chrome something?

I also added `Environement.ips`. If `--ips` is provided, only the given ips can discover the leapcast.
That's because I have roomates, and I just don't want them to start ridiculous Youtube videos on my computer :-)

I don't really like what I did with `buf` in `leapcast/services/websocket.py`, but it's a quick hack to make sure we don't just discard some messages if a websocket is not yet open.

`requests` is also a dependency of leapcast.


I mostly just hack my way around to get Pandora working (like with `buf`). If you want to refactor a few things, I would be happy to do it (I'll be traveling next week, but I should have some time after that).